The success of the modem semiconductor industry largely relies upon lithography, the art of transferring a given circuit pattern to a semiconductor substrate. Several techniques are available for lithography depending upon the type of radiation used for the pattern transfer, for example, optical, x-ray, UV, focused ion, and electron beam radiation. The wavelength of the radiation that is employed, as well as the optics involved, dictate the smallest feature size (e.g., the “Critical Dimension” or “CD”) in the circuit that can be transferred during the lithographic process.
Electron beams (ebeams), unlike all other radiation types, can be easily accelerated to energy levels of greater than 20 kV. As a result, it is possible to achieve ebeams with wavelengths of less than 10 pm. For this reason, lithographic processes employing ebeams are quite desirable in theory. However, conventional lithographic processes employing focused electron beam radiation are limited insofar as the electron beams can only be generated by way of narrow point sources, for example, by way of a hot filament or Spindt type field emission tip such as that disclosed in U.S. Pat. No. 3,665,241, which is hereby disclosed by reference herein.
Due to the narrow width of ebeams used in conventional lithography techniques, it is often possible to achieve CDs of approximately 1 nm. Yet due to the narrow width of the ebeams, conventional lithography processes also require special procedures to allow relatively large chips to be generated using the narrow ebeams. For example, in some conventional embodiments, the lithography process involves an ebeam writer that moves the narrow ebeam in relation to a mask/wafer in a rasterizing manner. However, such a serial tool, also known as direct-write tool, is unsuitable for high volume, commercial semiconductor manufacturing.
To address these problems, several techniques that utilize multitudes of narrow ebeams in parallel configurations have also been developed. Among these techniques, for example, are those disclosed in U.S. Pat. Nos. 6,333,508; 6,614,035; 6,175,122; and 6,194,732 as well as in B. J. Kampherbeek et al., “An experimental setup to test the MAPPER electron lithography concept,” Microelectronic Engineering 53: 279-282 (2000), and C. David et al., “Low energy electron proximity printing using a self-assembled monolayer resist,” Microelectronic Engineering 30: 57-60 (1996), each of which is hereby incorporated by reference herein.
The electron optics developed for all these techniques based on conventional point sources limits the maximum usable beam diameter (and optical field) to <1 mm as opposed to several cm of optical field in UV, x-ray and visible radiation. Therefore, to expose an entire wafer with several dies, one requires precise mechanical movement of the wafer stage, accurate registration, and stitching of multiple optical fields. These complex processes increase the cost of the lithographic tools. In addition, stochastic electron-electron interactions limit the maximum fluence achievable in a small optical field (<1 mm) for a given CD, thereby limiting the wafer throughput.
Because of the limitations associated with using point sources to generate electron beams, various efforts have been made to develop other types of sources for electron beams. Recently, various devices allowing field emission from planar structures such as MIM (Metal Insulator Metal) devices and MOS (Metal Oxide Semiconductor) devices have been developed. One subclass of devices within the broader class of MOS devices are Metal-Porous Silicon-crystalline Silicon structures known as PS diodes, such as are disclosed in U.S. Pat. Nos. 6,844,664; 6,815,875; 6,187,604; and 6,426,234, which are hereby incorporated by reference herein. Several variations of this type of electron source are available in the public literature as well.
Although the aforementioned planar structures offer the capability of outputting wider beams of electrons, each of these conventional devices suffers from several problems. In particular, these problems include poor reproducibility of emission characteristics over a large area of the device as well as from device to device, a short working life, and a large spread in the energy spectrum of the emitted electrons. These technical problems prevent the devices from being useful in lithography as well as other high end applications such as microscopy.
Therefore, it would be advantageous if a new or improved system for generating focused electron beams could be developed that allowed for the generation of electron beams having greater beam width. Further, it would be advantageous if such a new or improved system employed a source that did not suffer from the same disadvantages as conventional planar PS diode sources or the like.